1. Field of the Invention
The present invention relates to a load control device comprising a bidirectional semiconductor switch for controlling the amount of power delivered to an electrical load, and more specifically, to a method of detecting a fault condition of the load control device, in particular, a failure condition of the bidirectional semiconductor switch.
2. Description of the Related Art
Bidirectional semiconductor switches are typically used in various load control devices to control the amount of power provided from an alternating current (AC) power source to a load, such as a lighting load. A bidirectional semiconductor switch may comprise one semiconductor switch such as a field effect transistor (FET) within a full-wave rectifying bridge or a triac; or two semiconductor switches, such as two FETs or two insulated gate bipolar transistors (IGBTs), coupled in anti-series electrical connection, or two silicon-controlled rectifiers (SCRs) coupled in anti-parallel electrical connection.
The load control device may be operable to either switch (i.e., turn on and off) or dim (i.e., adjust the intensity of) the controlled load. In order to dim a lighting load, the load control device may employ a phase control dimming technique such that the bidirectional semiconductor switch is rendered conductive for a portion of a given half cycle of the AC power source and non-conductive for the remaining portion of the given half cycle, thereby providing only a portion of the AC power source to the load. For example, the load control device may comprise two FETs coupled in anti-series connection, such that each FET can be independently rendered conductive and non-conductive to provide the appropriate amount of power to the load (i.e., the first FET is rendered conductive during the positive half cycle of the AC power source and the second FET is rendered conductive during the negative half cycle). Additionally, when a load control device employs the phase control dimming technique, the load control device will typically control the bidirectional semiconductor switch to be conductive for the same amount of time in the positive half cycle and in the negative half cycle so as to provide a symmetric AC voltage waveform to the load.
Semiconductor switches, like many electrical components and devices, can be subject to failures. The failure of the semiconductor switch may be the result of an exposure to over-voltage, over-current, and/or over-temperature conditions, or simply the expiration of a rated lifetime of the semiconductor switches. One type of electrical failure (or fault condition) is defined as a semiconductor switch failing in a shorted state (i.e., fully conductive state).
If the load control device is using a phase control dimming technique to control two anti-series coupled semiconductor switches, and one of the semiconductor switches has failed in the shorted state, it is possible that the load control device will provide an asymmetric AC voltage waveform to the load. For example, the asymmetric AC voltage may be caused when one of the semiconductor switches is rendered non-conductive during a portion of a positive half cycle but the other semiconductor switch is fully conductive during the entire negative half cycle as a result of failing in the shorted state. The asymmetric AC voltage provided to the load results in a direct-current (DC) offset that may create undesirable conditions for certain load types. For example, the asymmetric AC voltage can cause a magnetic low voltage load having a transformer to saturate, overheat, and potentially damage the transformer and/or the load control device.
Thus, it is advantageous for a load control device to be able to detect an asymmetric voltage condition across the load, and more particularly, to detect when one or more of the semiconductor switches of the bidirectional semiconductor switch has failed in a shorted state. Upon detecting the failure, the load control device may initiate a protection mode, such as, for example, driving the other (properly-functioning) semiconductor switch to a fully conductive state as well. When one of the semiconductor switches is electrically shorted and the other semiconductor switch is controlled to be fully conductive, the resulting full AC waveform provided by the load control device to the load is symmetrical, and there is no DC offset. Thus, the protection mode can mitigate potential damage to certain load types and/or to the load control device itself.
However, the failure detection methods of prior art load control devices would sometimes result in false detections of failure conditions. For example, certain load conditions would be incorrectly detected as an indication of a failed semiconductor switch, and thus, the load control device would initiate the protection mode unnecessarily and provide the full AC waveform to the load. In other words, the load control device would no longer dim the intensity of the lighting load while operating in the protection mode.
Therefore, there is a need for a more reliable and improved method of detecting a fault condition of a load control device wherein the fault condition may comprise an asymmetry condition, or more particularly, a failure condition of a semiconductor switch.